Superabsorbants with controlled absorption speed

ABSTRACT

The invention relates to hydrophilic, highly swellable polymers for aqueous fluids, e.g. superabsorbants, which are coated with reactive, water-insoluble, film-forming, hydrophobic polymers and an additional reactive constituent which can react with carboxyl groups of carboxylate anions and form additional cross-links on the particle surface. The invention also relates to the production of superabsorber thus coated and their use in hygiene articles used to absorb body fluids. The products obtained by coating highly swellable, hydrophilic, insoluble polymers with reactive, water-insoluble, film-forming, hydrophobic polymers and an additional reactive component are characterized in that their absorption speed cam be adjusted in an controlled manner extended area according to the type and amount of reactive, water-insoluble, film-forming, hydrophobic polymers used in the coating and according to process engineering conditions chosen for said coating.

The invention relates to hydrophilic, highly swellable polymers foraqueous liquids, so-called superabsorbers, which are coated withreactive, water-insoluble, film-forming, hydrophobic polymers and anadditional reactive component capable of reacting with carboxyl groupsor carboxylate anions to form additional crosslinking sites on theparticle surface, the production of such coated superabsorbers and theiruse in hygienic articles used for absorbing body fluids.

Highly swellable polymers which may be obtained by polymerization ofunsaturated acids such as acrylic acid, methacrylic acid,acrylamidopropanesulfonic acid, etc., or partially neutralized in theform of the alkali or ammonium salts thereof, in the presence of smallamounts of polyunsaturated compounds are already known.

Likewise, superabsorbent crosslinked polymers which may be obtained bygraft copolymerization of unsaturated acids onto various matrices, suchas polysaccharides, polyvinyl alcohols, polyalkylene oxides andderivatives thereof, are well-known. The highly swellable polymersmentioned are remarkable for their ability to absorb large quantities ofaqueous liquids such as blood or urine with swelling and formation ofhydrogels and retain the absorbed quantity of liquid even underpressure.

As a result of their characteristic absorbing properties, these polymersare preferably used as absorbents in hygienic articles.

According to the state of the art, the properties of these hydrogels canbe modified by a surface treatment using specific substances. To thisend, conventional hydrogels which have been dried, milled and optionallyscreened, are reacted in the form of powders with reactive compounds,i.e., compounds containing groups capable of reacting with the carboxylgroups of the hydrogels to give covalent crosslinking on the surface ofthe granulate particles. These reactive compounds may be di- or polyols,bisepoxides, higher epoxides, or cyclic carbonic esters as well.

Such surface crosslinking has been described in DE 40 20 780 C1, forexample, where alkylene carbonates are employed as surface crosslinkers.

The surface treatment of hydrophilic, highly swellable polymers usingreactive substances for the purpose of surface crosslinking has beendescribed in numerous other publications.

EP-A 349,240 (to achieve balance between absorptive capacity andabsorption rate as well as gel stability and absorbency) describes anaftertreatment of polymers using crosslinking agents including two ormore functional groups, which may undergo reaction with the carboxyl orcarboxylate groups or other groups contained in the polymer.Conventionally, the hydrophilic, highly swellable powdered polymer isdirectly mixed with one or more reactive hydrophilic components,optionally using water and organic solvents.

Inter alia, EP-B 317,106 describes di- or polyols, di- or polyglycidylethers, haloepoxy compounds, polyisocyanates, polyamines, polyfunctionalaziridine compounds, or alkyl di(tri)halides as reactive,surface-crosslinking components.

Surface subsequent crosslinking is effected by performing a thermaltreatment of the products subsequent to coating the superabsorbentpolymer powders.

The above-described procedures have in common that a significantimprovement of the absorptive capacity under pressure for aqueousliquids such as urine, blood or other body fluids is achieved throughthe thermal treatment of the highly swellable, powdered polymers in thepresence of the reactive components mentioned.

The surface treatment of granulated, hydrophilic, highly swellablepolymers using additives for dedusting the granulated polymers, e.g.,using polyols or polyethylene glycols in PCT/US93/02872, is alsowell-known. The laid-open document DE 44 26 008 A1 refers to the use ofnon-reactive, water-insoluble, film-forming polymers of, inter alia,homo- and copolymerizates of acrylic and methacrylic esters, vinylesters, polyamides, polyesters for dedusting and improving the abrasionresistance. For the same purpose of improving the abrasion resistance,but also in order to reduce the caking tendency, the DE 195 24 724 A1describes a surface treatment using non-reactive, water-soluble waxes.DE 44 14 117 A1 describes the use of non-reactive polysiloxanes, e.g.,polydimethylsiloxanes and polymethylphenylsiloxanes for the surfacetreatment of hydrophilic, highly swellable polymers for use in dedustinga superabsorbent polymer.

EP-A 705,643 A1 refers to water-absorbing polymers having improvedproperties, which are obtained by treating crosslinked polymers based onacrylic acid or the salts thereof with a modified silicone oil havingfunctional groups capable of reacting with carboxyl groups and/orcarboxylate groups, with amino and epoxy-functional silicones beingmentioned as examples. The silicones may be applied as substance, as asolution in an organic solvent, or as an emulsion. If necessary, thepolyacrylate/silicone blend may be postcured at temperatures of 60-200°C.

The products obtained using the described functional silicone oils andthe described process have improved anti-caking performance and areduced dusting tendency. In particular, the absorption under pressureis markedly improved by the treatment described. The products have goodinitial absorption under pressure and absorptive capacity. The Examples1 through 4 used in EP-A 705,643 show that the absorption of theproducts including functional silicone oil is higher both prior to andafter postcure of the materials after 10 minutes and thus, theabsorption rate is higher compared to the material according toComparative Examples 1 through 8, which has not been surface-treated.

Therefore, the absorbent polymers from EP-A-705,643 also belong to thosesuperabsorbers that are conventional in this respect, tending to rapidabsorption of an aqueous liquid immediately upon contact with such aliquid, giving rise to rapid swelling near the entry site and so-calledgel blocking.

Likewise, methods and additives for improving the absorption rate ofsuperabsorbent hydrophilic polymers have been known for a long time.

DE 44 18 818 A1 describes the addition of, inter alia, CO₂, alkaline,alkaline earth and ammonium carbonates and bicarbonates to the monomersolution as expanding agent, in order to enhance the absorption rate ofhighly swellable polymers.

The U.S. Pat. No. 4,548,847 describes hydrogels reversibly crosslinkedby at least bivalent metal cations, such as Ca²⁺ or Ba²⁺. By co-using asubstance capable of removing the metal cations, it is possible toachieve delayed swelling. Water-soluble chelating agents such asNa₂HPO₄, sodium hexametaphosphate and the disodium salt ofethylenediamine tetraacetate are preferably used. These substances causethe reversible crosslinking sites formed by said at least bivalent metalcations to be degraded by complexing. The dropping crosslinking densitypermits stronger swelling of the products. The patent also describesabsorbent articles such as wound dressings and tampons which contain theabsorbent polymers having delayed swelling characteristics. Among otherthings, an absorbent article (wound dressing) made up of layers isdescribed, wherein the polymer of the invention is contained at certainpoints in each layer. Combinations of the polymer according to theinvention with other polymers absorbing aqueous liquids are notmentioned.

The patent applications GB 2,280,115 A and WO 95/00183 describe anabsorbent article containing covered superabsorbent particles in thatarea into which body fluid is released. The cover of the superabsorbentparticles prevents swelling thereof until the cover has dissolved in thetest liquid or body fluid or is penetrated by same. Thus, they aresuperabsorbent particles having an activation period until swellingbegins, which period can be varied by means of the cover material andthe thickness thereof. Non-reactive polysaccharides such as gelatin,micro-crystalline cellulose and cellulose derivatives are mentioned ascover materials. The activation period until swelling begins should beat least five, preferably 15, and more preferably 60 minutes.

The application FR 2,627,080 A1 also describes absorbent articlescontaining multiple superabsorbers which exhibit a swelling behaviorwith varying delay. The superabsorbers described consist ofpolyacrylates provided with a gradually dissolving cover.

Superabsorbers involving such surface treatment are disadvantageous inthat even minor initial wetting—without necessarily inducing swelling ofthe highly swellable polymer by itself—will result in a deterioration ofthe treated surface by dissolving or peeling, swelling or decomposition.Once the cover of the polymer has dissolved, these superabsorbers have aswelling rate like that of a conventional material which has not beensurface-treated. As a result, the desired effect of improved liquidmanagement in the absorbent article is lost.

Superabsorbers having a slow, preferably constant swelling rate withoutthe behavior of delayed initial swelling and their use in absorbentarticles have not been described in GB 2,280,115 A, WO 95/00183 and FR2,627,080 A1.

The patent application EP 0,631,768 A1 describes an absorbent article inwhich as well, superabsorbers having varying absorption rate are used.The differences between the various superabsorbers used are due to adifferent grain size distribution (type 1: 600-850 μm, type 2: <250 μm)and are accordingly small. Surface-treated superabsorbers and their usein absorbent articles have not been described therein.

The development towards thinner and thinner diaper constructions in thefield of hygienics is associated with a higher compacting of theabsorbent core and an increase of the superabsorber ratio in thesuperabsorber/cellulose mixture. As a result, the homogeneous liquiddistribution within the absorbent core becomes a more and more importantcriterion for full-scale utilization of the storage capacity of thehighly swellable polymer.

The reduction of the cellulose amount in the absorbent core has anadverse effect on the liquid distribution. Meanwhile, the use ofadditional means such as special fleeces in order to optimize the liquiddistribution has become state of the art.

Additional demands with respect to the liquid management are made on thehighly swellable, liquid-storing polymers as well. Even in asuperabsorber/cellulose mixture having a high ratio of highly swellablepolymers, the liquid-storing polymers must permit or support rapiddistribution of liquid within the superabsorber/cellulose mixture.

Conventional superabsorbers which are frequently optimized for a highliquid absorption rate in particular, have a tendency to absorb andstore an aqueous liquid immediately upon contact with such a liquid,which is associated with a massive volume increase of the polymerparticles.

In the vicinity of the entry site of body fluid into the absorbent core,the absorbent polymers undergo massive swelling as a result of theirhigh absorption rate for aqueous liquids, because the distribution ofliquid is slower than the liquid storage by the highly swellable polymeras a result of the low cellulose ratio in the absorbent core. Owing tothis rate difference, a major part of the liquid is absorbed in theimmediate vicinity of the entry site.

Moreover, this effect is reinforced in that the swelling of aconventional superabsorber takes place in such a way that immediatelyupon addition of liquid, a very high absorption rate for aqueous liquidsis observed, in association with a steep increase of absorption. After afew minutes already, a highly swellable polymer based on crosslinked,partially neutralized polyacrylate has reached about 95% of itsabsorptive capacity under conditions of free swelling. Thereafter, theamount of absorbed liquid asymptotically approaches its equilibriumvalue. As a result, an excessive proportion of liquid per unit time ittaken up by the described highly swellable polymers immediately afteraddition of the liquid to be absorbed. This behavior is a typicalmaterial property of crosslinked polyacrylates.

As a result of the expansion of the polymer particles associated withthe absorption of liquid, the interstices and pores of the SAP fluffmatrix close within the absorbent core area around the entry site of thebody fluid. As the liquid transport through a swelled hydrogel bydiffusion is some magnitudes slower compared to the flow in interstices,blocking occurs in this area. This effect is frequently referred to asso-called “gel blocking” in literature. Succeeding amounts of body fluidcannot penetrate the absorbent core and will be transported randomlyover the surface of the already surface-saturated area up to the edge ofsame.

As a consequence, the reverse wetting behavior and leakage behavior aredeteriorated. In addition, the storage capacity of the absorbent core isreduced because highly absorbent polymers embedded deeper in theabsorbent core can no longer be reached by additional body fluid fromlater dosages as a result of swelling of the particles at the surfaceand thus, cannot contribute to the overall storage capacity.

It is therefore the object of the present invention to provide highlyswellable polymers wherein the rate of liquid absorption can be adjustedover a wide range in a controlled fashion.

It is another object of the present invention to provide highlyswellable polymers wherein the liquid absorption preferably proceeds ata constant rate over a wide range.

It is another object of the present invention to provide highlyswellable polymers having controlled absorption rate and sufficientflowability, which may be used in absorbent hygienic articles.

It is also an object of the present invention to provide highlyswellable polymers which, by controlled adjustment of the liquidabsorption rate, optimize the above-described unfavorable and thus,undesirable relationship between swelling rate of the superabsorber anddistribution rate of body fluid in the absorbent core for aqueousliquids.

Moreover, it is an object of the present invention to achieve moreeffective utilization of the overall storage capacity of the amount ofhighly swellable polymer contained in the respective hygienic article.

In addition, it is an object of the present invention to reduce theadverse effect of gel blocking on the liquid distribution in theabsorbent core.

It is still another object of the present invention to provide a processfor producing highly swellable polymers having controlled absorptionrate and sufficient flowability.

Surprisingly, it has been found that highly swellable polymers absorbingaqueous liquids, which polymers are constituted of monoethylenicallyunsaturated monomers bearing acid groups and optionally, other monomerscopolymerizable therewith and optionally, water-soluble polymerssuitable as basis for grafting, and polyunsaturated monomers acting ascrosslinkers, exhibit the above-described desirable swelling behavior,namely, a controlled retardation of the absorption process controllableas to its degree, in association with a constant swelling rate, by beingcoated with a reactive, water-insoluble hydrophobic polymer and anotherreactive component capable of reacting with carboxyl groups orcarboxylate ions to form additional crosslinking sites on the particlesurface, and postcured at a specific temperature.

It has also been found that using hydrophobic polymers to be usedaccording to the invention together with another reactive componentcapable of reacting with carboxyl or carboxylate groups, therebyresulting in additional crosslinking on the surface of the highlyswellable, hydrophilic polymer particles, e.g., ethylene carbonate orpolyhydric alcohols (glycerol etc.), has a substantially higher impacton the swelling rate as is the case without post-crosslinking agent. Asa rule, quantities of from 0.1 wt.-% to 1 wt.-% of post-crosslinkingagent and between 0.005 and 2% of the hydrophobic polymer of theinvention are used in surface post-crosslinking.

Thereby, products having a controlled absorption rate for aqueousliquids can pe produced, the flowability of which being within a rangethat permits machine processing.

Accordingly, the present invention relates to a hydrophilic, highlyswellable, powdered polymerizate, characterized in that it is coatedwith a reactive, water-insoluble polymer and subsequently treated atelevated temperature in the presence of a post-crosslinking agent, sothat as a consequence of the described treatment, a controlledretardation of the absorption rate compared to a standard product nottreated with reactive silicones results, without a drop in flowabilityof the highly swellable polymer of the invention having controlledabsorption rate below a limit of 8 g/s.

Therefore, the invention is directed to swellable polymers absorbingaqueous liquids, characterized by being constituted of

a) monomers bearing monoethylenically unsaturated acid groups,

b) optionally, other monomers copolymerizable therewith, and

c) optionally, water-soluble polymers suitable as basis for grafting,and

d) at least diunsaturated monomers acting as crosslinkers, characterizedin that the polymers have been treated with

e) a reactive, water-insoluble hydrophobic polymer, and

f) another reactive component capable of reacting with carboxyl groupsor carboxylate anions to form additional crosslinking sites on theparticle surface, and postcured.

The components a) through d) are contained in the corresponding polymerin the following amounts, relative to component a):

a) 60-99.99 wt.-%, preferably 90-99.9 wt.-%;

b) 0.1-35 wt.-%, preferably 0.5-20 wt.-%;

c) 0-30 wt.-%, preferably 0-20 wt.-%;

d) 0.01-3.0 wt.-%, preferably 0.05-1.0 wt.-%.

The amounts indicated refer to the weight of dry polymer.

Coating the highly swellable polymers with the hydrophobic polymer to beused according to the invention may be performed using the puresubstance; however, the hydrophobic polymer to be used according to theinvention may also be applied as a solution in a suitable organicsolvent, such as chloroform, toluene, tetrahydrofuran, etc.

Suitable, chemically or physically reactive, water-insoluble,hydrophobic polymers are those which are thermally stable in atemperature range up to 250° C., forming water-insoluble, yetwater-permeable polymer films at the surface of the highly swellablepolymers absorbing aqueous liquids.

Furthermore, suitable reactive, water-insoluble, hydrophobic polymersare those forming polymer films which have no tendency of agglutinatingwithin a temperature range of between 0° C. and 250° C.

Finally, suitable polymers are those, in particular, which themselvesinclude functional groups that undergo physical and/or chemicalinteractions with the acid groups or carboxylate groups of the highlyswellable polymers absorbing aqueous liquids, and thereby provide goodspreading on the surface of same and remain fixed thereon even uponwetting with water.

Suitable hydrophobic polymers are homo- and copolymerizates ofpolysiloxanes having randomly distributed monomer units includingsecondary and additional primary amino groups which, as functionalgroups, undergo interaction with acid groups. The interaction may be achemical bond or an electrostatic interaction.

Hydrophobic polysiloxanes preferably used according to the invention arepoly[dimethyl-co-aminoalkylmethyl-co-methyl(polyether)]siloxanes andpoly[dimethyl-co-aminoalkylmethyl]siloxanes in accordance with formula1.

wherein

n=50-99

m=1-50

k=1-11

i=1-12, and

R, R′ represent H, alkyl, preferably methyl, hydroxyalkyl, aminoalkyl.

In addition, polydimethylsiloxanes, polydialkylsiloxanes as well aspolyalkylarylsiloxanes and polydiarylsiloxanes having amino groups atthe terminal siloxane units of the polysiloxane main chain arepreferred.

Particularly preferred hydrophobic polymers to be used according to theinvention are those amino-functional polysiloxanes having improvedpolymer emulsifiability in aqueous media as a result of polyether sidechains at the polysiloxane backbone. Due to the modification of theextremely hydrophobic polysiloxane by hydrophilic polyether side chains,such as polyethylene oxide, these polysiloxanes, depending on thepolyethylene oxide content relative to the amount of polysiloxane, turninto auto-emulsifying systems which then may be used together with thepost-crosslinking agent in aqueous emulsions.

The hydrophobic polymers to be used according to the invention may belinear or branched. The viscosity of the polymers may range between 350and 10,000 mPa·s. Mixtures of different polysiloxanes may be used.

By employing the hydrophobic polymers to be used according to theinvention in combination with a second reagent resulting in additionalcrosslinking on the particle surface, e.g., di- or polyols, ethylenecarbonate, or bis- or polyepoxides, the absorption rate of the highlyswellable, crosslinked polyacrylates may be varied in a well-aimedfashion, with appropriate experimental conditions, by varying the amountof hydrophobic polymer. Advantageously, the time dependence of theabsorption without external pressure can be adjusted in such a way that70% of the absorptive capacity of the superabsorbent polymer is notreached already after 10 minutes but only after at least the 1.5 fold ofsaid time period (cf., evaluation of FIG. 1 and/or Table 1).

Table 2 illustrates the time in minutes which is required for productsof the invention (Examples 1 through 8) and prior art comparativeproducts (V1 through V4) to reach 70% absorptive capacity, as well asthe time factor by which said swelling period is extended relative tothe comparative product V1 as a conventional superabsorber (referencesubstance).

The hydrophobic polymers to be used according to the invention areemployed in amounts of between 0.005 wt.-% and 2 wt.-%. Preferably,between 0.01 wt.-% and 1.5 wt.-%, and more preferably between 0.05 and0.8 wt.-% of modified, functional polysiloxane is employed.

Since surface coatings using polysiloxanes or other polymers such aspolyethylene oxide, polyalkyl (meth)acrylates or polyamides reduce theflowability of the highly swellable polymers, it is not possible toapply unlimited quantity ratios as coating.

Rather, the amount of hydrophobic polymer used has an upper limit due tothe flowability of the absorbent polymer granulates required for machineprocessing. Desirably, the flowability of the absorbent polymers is asgood as possible. Products having a flowability of less than 8 g/s(measured using a funnel 10 mm in width at the outlet) are undesirablein technical terms.

Coating can be performed using a single hydrophobic substance or anymixture of the hydrophobic polymers described.

In a particularly preferred embodiment, the reactive hydrophobic polymerto be used according to the invention may be applied in its protonatedform after neutralization of the amino groups with mineral acids ororganic acids, emulsified or dissolved in water or organic solvent/watermixtures (Example 8).

The invention also relates to a process for producing the inventiveswellable polymers absorbing aqueous liquids, characterized in that aswellable polymer absorbing aqueous liquids, constituted of

a) monomers bearing monoethylenically unsaturated acid groups,

b) optionally, other monomers copolymerizable therewith, and

c) optionally, water-soluble polymers suitable as basis for grafting,and

d) at least diunsaturated, powdered monomers acting as crosslinkers, iscoated with at least

e) one reactive, water-insoluble hydrophobic polymer, and

f) at least one other reactive component capable of reacting withcarboxyl groups or carboxylate anions to form additional crosslinkingsites on the particle surface, and subsequently subjected to a thermaltreatment in a temperature range of from 80 to 230° C., preferably from170 to 200° C.

As reactive component f) capable of reacting with carboxyl groups orcarboxylate anions to form additional crosslinking sites on the particlesurface, di- or polyols, bisepoxides, higher epoxides, polyamines,polyamidoamines, or cyclic carbonic esters are preferably used,typically in amounts of from 0.1 to about 1 wt.-%, relative to thepolymer from a) through d). Each of these surface crosslinking agentsmay react with at least 2 carboxyl groups of the hydrogels of theswellable polymer from a) through d) with covalent crosslinking on thesurface of the granulate particles.

To this end, one or more of the reactive compounds mentioned aredissolved in water and/or an organic solvent and applied onto thesurface of the dried, hydrophilic, highly swellable polymerizate.Suitable mixing units for applying the post-crosslinking agent are,e.g., Patterson-Kelley mixers, DRAIS turbulence mixers, Lödige mixers,Ruberg mixers, screw mixers, pan mixers, and fluid-bed mixers, as wellas continuously operated vertical mixers wherein the powder is mixed ata rapid frequency using rotating knives (Schugi mixer). Where theproduction of the highly swellable polymers is performed according tothe suspension polymerization process, the post-crosslinking agent mayalready be added to the suspension, i.e., prior to isolation of thepolymer.

Subsequently, the polymerizate powder is subjected to a thermaltreatment at temperatures of from 80 to 230° C., preferably between 170and 200° C. The time period for additional heating is limited by thatpoint where the desired properties pattern of the superabsorber isdestroyed as a result of heat damage.

Coating of the highly swellable polymer using a multifunctional reactivecompound resulting in an increase of the crosslinking density at thesurface under elevated temperature and thus, in improved properties ofthe highly swellable polymer in absorption under pressure (e.g.,post-crosslinking using ethylene carbonate, di- or polyols, bis- orpolyepoxides) may be effected prior to (Examples 1-5, 7), together with(Examples 6 and 8) or subsequent to applying the hydrophobizing agentonto the highly swellable polymer. Likewise, heating duringpost-crosslinking required for improving the absorption properties underpressure may be performed prior to (Example 7), subsequent to ortogether with (Examples 1-6, 8) the postcuring step resulting infixation of the hydrophobic polymer.

Where coating of the highly swellable polymers with the reactivehydrophobic polymers to be used according to the invention is performedaccording to the above-mentioned thermal treatment for improving thepolymerizate absorption properties under pressure, a second postcuringprocedure normally follows in order to fix the reactive hydrophobicpolymer at the surface of the highly swellable polymerizate. Optionally,coating the superabsorber powder with hydrophobic polymer may also becarried out together with the surface crosslinking agent as an aqueousemulsion or simultaneously using separate dosage means in a singleprocessing step prior to the subsequent heating step.

The postcuring step may be repeated several times. Suitable mixing unitsfor mixing the hydrophobic reactive polymer with the highly swellablehydrophilic polymerizate include the above-mentioned screw mixer, Schugimixer, etc.

In particular, it is possible to coat the hydrophobic polymer and themultifunctional compound successively onto the powdered, highlyswellable polymer and carry out the necessary postcuring for surfacepost-crosslinking and simultaneous hydrophobizing in a single step(Examples 1-5).

In a particularly preferred embodiment of the process, themultifunctional compound and the hydrophobic polymer in the form of itssalt are dissolved or emulsified in water and applied together as anacidic solution onto the superabsorbent polymer (Example 8).

Suitable for postcuring and performing the actual post-crosslinkingreaction are, e.g., belt dryers, hot-air dryers or blade dryers.

The hydrophobizing agent to be used according to the invention may beapplied as substance, as a solution in a suitable organic solvent, suchas chloroform, toluene, tetrahydrofuran etc., or in its protonated formafter neutralization of the amino groups with mineral acids or organicacids, emulsified or dissolved in water or organic solvents.

The highly swellable polymers of the invention having controlledabsorption rate may be used in hygienic articles for absorbing bodyfluids.

Accordingly, the invention is also directed to the use of the highlyswellable polymers of the invention in hygienic articles for absorbingbody fluids.

Hydrophilic, highly swellable, powdered polymerizates to be usedaccording to the invention are homo- and copolymerizates based onunsaturated carboxylic acids and/or derivatives thereof.

The unsaturated carboxylic acids and derivatives are exemplified byacrylic acid, methacrylic acid, itaconic acid and maleic acid and thealkaline, ammonium and amine salts or the amides thereof, such as(meth)acrylamide, N-tert-butyl(meth)acrylamide andN-isopropyl(meth)acrylamide, and acrylonitrile.

It is also possible to use other comonomers such as olefinic unsaturatedsulfonic acids. Examples include the salts of2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid,vinylsulfonic acid, allyl and methallylsulfonic acid, especially thealkaline, ammonium and amine salts thereof.

As further comonomers, the above-mentioned copolymerizates based onunsaturated carboxylic acids and derivatives thereof may includepolyglycol esters of unsaturated acids, e.g., polyalkylene oxide(meth)acrylates of general formula 2:

wherein

R¹=hydrogen or methyl,

n=2-50, and

R²=hydrogen or an aliphatic, araliphatic or cycloaliphatic C₁-C₁₂residue, e.g., methyl, ethyl or butyl.

The hydrophilic, highly swellable polymerizates forming the basis of thehydrogels of the invention are crosslinked, i.e., contain comonomershaving at least two double bonds that are polymerized into the polymernetwork.

Suitable crosslinkers are esters of unsaturated carboxylic acids ofpolyols, such as ethylene glycol diacrylate, di-, tri- or polyethyleneglycol diacrylate or methacrylate, butanediol diacrylate or methacrylateand trimethylolpropane triacrylate or methacrylate, ethoxylatedtrimethylolpropane triacrylate or methacrylate derivatives,pentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritoltetra(meth)acrylate derivatives and allyl compounds such as(meth)allylpolyethylene glycol(meth)acrylate, tetraallyloxyethane,triallylamine, tetraallylammonium chloride, triallylmethylammoniumchloride, and methylenebisacryl- or methacrylamide. These crosslinkersare normally used alone or in combination in amounts of from 0.05% to1%.

The hydrophilic, highly swellable polymerizates forming the basis of thehydrogels of the invention may be produced using well-knownpolymerization processes. Predominantly, the polymerization is carriedout in aqueous solution according to the process of the so-called gelpolymerization. Other processes such as suspension polymerizationprocesses are also common.

Therein, a solution in water is produced including the monomers,crosslinker and optionally other additives, such as polyvinyl alcohol oroligo- or polysaccharides such as starch or modified starch as anappropriate grafting basis. Such polymers and processes for theirproduction are well-known in literature; for example, U.S. Pat. No.4,076,663 describes the use of starch, and GB 1,490,128 polyvinylalcohol and starch as a basis for grafting.

The monomer solution is neutralized to an extent of at least 25 mole-%,preferably at least 50 mole-%, and more preferably 50-80 mole-% assodium, potassium or ammonium salt.

Conventionally, the polymerization is initiated using water-solublefree-radical initiator systems at temperatures of <15° C. Asfree-radical initiator systems, both redox systems such as ascorbic acidor sodium sulfite with water-soluble peroxides (e.g., hydrogen peroxide,tert-butyl hydroperoxide) and thermal initiators such asazobis(2-amidinopropane) dihydrochloride or persulfates are suitable.Preferably, a combination of a redox initiator system and thermalcatalysis is used.

Once started, the polymerization proceeds to virtually quantitativeconversion, utilizing the Trommsdorff-Norrish effect. The reaction isconducted adiabatically, the reaction medium may reach temperatures ofup to 120° C., and the reaction may be carried out both at normalpressure and elevated pressure.

It is preferred to perform the polymerization under a protective gasatmosphere, preferably nitrogen.

The gel formed is subsequently crushed, dried, milled and screened tothe desired particle size.

To improve the absorption under pressure, the polymerizates thusobtained are reacted in a powdered form with surface crosslinkers, i.e.,compounds containing groups each of which can react with at least twocarboxyl groups of the hydrogels to give covalent crosslinking on thesurface of the granulate particle. Preferably, di- or polyols,bisepoxides, higher epoxides, polyamines, polyamidoamines, or cycliccarbonic esters are used, as described in the above-mentioned patents.Typically, amounts of from 0.1 to about 1 wt.-% are used in surfacepost-crosslinking.

Testing Methods Retention

The retention is determined according to the tea bag method andindicated as average value from three measurements. About 200 mg ofsuperabsorber is welded in a tea bag and immersed in a 0.9% salinesolution for 30 minutes. Subsequently, the tea bag is suspended to drainfor 10 minutes. Thereafter, the tea bag is centrifuged in a centrifuge(23 cm in diameter, 1,400 rpm) for 5 minutes and weighed. A tea baghaving no superabsorber is run as a so-called blank. The retention iscalculated according to the formula:${Retention} = \frac{{{Final}\quad {weight}} - {Blank} - {{Initial}\quad {weight}}}{{Initial}\quad {weight}}$

and is indicated in g/g.

Solubles

The solubles are determined as described in U.S. Pat. No. 4,654,039,with the exception that a 0.9% saline solution is used as test liquidinstead of synthetic urine.

Flowability

The flow rate of superabsorbent polymers is determined by allowing arepresentative sample (A)=100 g to flow through a flow funnel (accordingto DIN 53492 at an inclination angle of 40°, and a nozzle according toDIN 53492, having an orifice of 10 mm±0.1 mm) into a vessel ofsufficient volume. Initially, the orifice at the bottom side of thefunnel must be closed. The sample is cautiously poured into the funnel.The vessel is placed underneath the orifice of the funnel. The funnelmay not be exposed to shocks and concussions. Once the orifice has beenopened, the time (F) required by the sample to pass through is measured,recorded with an accuracy of 0.1 g/s, and related to 1 g. The flow rateis given in g/s with an accuracy of 0.1 g/s.

In cases where the orifice has been opened and the sample does not flow,the sample is rated as non-flowing.${{Flow}\quad {rate}\quad \left( {g/s} \right)} = \frac{A\quad\lbrack g\rbrack}{F\quad\lbrack s\rbrack}$

Time-dependent Absorption with Free Swelling

500 mg±0.5 mg of superabsorber is weighed in a plexiglass cylinder 5 cmin diameter having a stainless steel screen bottom with a mesh width of43 μm. The accurate initial weight of the superabsorber (SAP initialweight) and the weight of the plexiglass cylinder including SAP (drygross weight) are recorded. The plexiglass cylinder including thesuperabsorber is placed on a zero porosity glass frit 120 mm in diameterin a bath containing 0.9 wt.-% saline solution, so that the screenbottom is situated at least 20 mm beneath the level of the salinesolution. In this way, it is ensured that the highly swellable,crosslinked polymer is immersed in a large excess of saline solution atany moment of measurement. When reaching the maximum swelling volume ofthe superabsorber in the plexiglass cylinder, the filling level of thesaline solution in the vessel must be sufficient so as to prevent theabsorbent gel from swelling beyond the level of the saline solution.

The measuring intervals may be selected arbitrarily; preferably, thecylinder was taken out of the bath after 5, 10, 15, 30, 60, and 120minutes in each case. It is possible to increase the swelling period to,e.g., 4 or 8 hours. The major part of non-absorbed saline solutioncontained in the plexiglass cylinder discharges through the screenfabric at the bottom, the remainder of non-bonded liquid is sucked offby placing the plexiglass cylinder on a suction flask fitted with arubber gasket (Ø=6 cm), which flask is under slight vacuum provided by awater suction pump.

Drops of liquid still adhering to the plexiglass cylinder after thenon-bonded saline solution has been sucked off are removed by placingthe plexiglass cylinder on domestic absorbent paper fleece.

The thus treated plexiglass cylinder including the swelled superabsorberis weighed with an accuracy of two decimal places (gross weight afterswelling) using an analytical balance and then immediately replaced intothe bath including the NaCl solution. The absorption upon free swellingis calculated according to the formula:${{Absorption}\quad (t)} = \frac{\left( {{Gross}\quad {weight}\quad {after}\quad {swelling}\quad (t)} \right) - \left( {{Dry}\quad {gross}\quad {weight}} \right)}{{SAP}\quad {inital}\quad {weight}}$

The numerical values thus obtained may be evaluated graphically byplotting the absorption (t) versus time t.

Fluff Combination Absorption Test (FCAT)

In this testing method, the swelling rate is determined bycomputer-aided recording of the absorption of a superabsorber/cellulosemixture under a pressure of 21 g/cm².

Preparation of the Cellulose Absorber Pads:

2.0000±0.0005 g of cellulose is weighed on an analytical balance. Thecellulose is used to form two or more cellulose plies. One cellulose plyis placed onto the analytical balance. Subsequently, e.g., 0.2000±0.0005g of superabsorber is sprinkled over the cellulose ply as uniformly aspossible. The second cellulose ply is placed on top, so that acellulose/SAP/cellulose sandwich is formed. In the preparation of thecellulose-SAP pads, the amount of SAP may be varied at will. Thesuperabsorber may also be sprinkled into several plies. The amount ofsuperabsorber used and the number of SAP plies are recorded. Preferably,an occupancy of 60%, i.e., 1.2 g of SAP on 2.0 g cellulose is used.

For the blank sample, a pad of 2.0000±0.0005 g of fluff is preparedwithout superabsorber.

Test performance:

The testing apparatus is illustrated schematically in FIG. 6.

The liquid level of the tubelet in the dropping funnel and the level ofthe testing apparatus table (discharge of liquid) should comply.

The prepared pad is placed on the testing apparatus in such a fashionthat the opening for the liquid is in a central position underneath thepad. The specimen is loaded with a weight corresponding to 21 g/cm².Higher loads are also possible.

To determine the overall absorption of liquid, the balance is tared to0.00 g. Measurement of the absorption curve is initiated by opening thecocks H1 and H2 and simultaneously starting the recorder connected tothe balance. The recorder automatically stores the weight detected bythe balance W. During measurement, the weight continuously increases asa result of the absorption of liquid. The blank is recorded in the samemanner using a cellulose pad (2.0000±0.0005 g) with no superabsorber.The amount of liquid absorbed by the blind sample is also recordedautomatically, and the values measured for the blind sample aresubtracted from those of the cellulose-SAP pad. Thereby, the absorptionbehavior of the pure superabsorber is obtained. By dividing thesenumerical values by the initial weight of superabsorber in grams, thetime-dependent absorption of the superabsorbers per gram ofsuperabsorber can be calculated.

The numerical values thus calculated may be illustrated graphically byplotting the absorption versus swelling rate.

Preparation of a Starting Material for Post-crosslinking andHydrophobizing Polymer A

A solution of acrylic acid, crosslinkers (0.2% trimethylolpropanetriethoxylate triacrylate and 0.3% a-allyloxypolyethylene glycolacrylate) and 2% methoxypolyethylene glycol methacrylate in water,partially neutralized with sodium hydroxide solution to an extent of 70mole-%, is polymerized by addition of initiators (15 ppm of ascorbicacid, 100 ppm of 2,2′-azobis(2-amidinopropane)dihydrochloride, 100 ppmof hydrogen peroxide solution, and 150 ppm of sodium peroxodisulfate)according to well-known procedures.

Once polymerization is completed, the gel-like polymer block is crushed,and dried at 150° C. for 120 minutes. Thereafter, the polymer is milledand screened for the grain fraction of 150-850 μm.

A highly swellable polymer having a retention of 38 g/g in 0.9% salinesolution is obtained. The polymer A further contains about 5%extractable components (measured after 1 h).

In the examples and comparative examples below, the followinghydrophobic polymers were used in the surface treatment of the polymer Astarting material:

NM 4266-750 silicone oil (commercial product of HÜLS Silicone GmbH,Nünchritz)

This is a random polysiloxane copolymer in accordance with formula 1,with n=99 and m=1, k=3 and i=2, having a viscosity of about 750 mPa·s.

NM 4266-1000 silicone oil (commercial product of HÜLS Silicone GmbH,Nünchritz)

This is a random polysiloxane copolymer having about 6 mole-% ofcomonomer units containing amino groups and a viscosity of about 1000mPa·s.

Magnasoft HSSD (commercial product of OSi Specialities)

The product is a amino-modified polyether silicone having about 0.25%nitrogen content and a viscosity of about 3700 mPa·s.

AP 500 silicone oil

The product is a polydimethylsiloxane and free of functional groups,having a viscosity of about 500 mPa·s.

EXAMPLE 1

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 0.15 g of NM 4266-750silicone oil is applied at 1000 rpm. The product thus treated ispostcured for 30 minutes at 190° C. in a circulating air oven. Theresults can be inferred from Table 1.

EXAMPLE 2

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 0.3 g of NM 4266-750silicone oil (commercial product of HÜLS Silicone GmbH, Nünchritz) isapplied at 1000 rpm. The product thus treated is postcured for 30minutes at 190° C. in a circulating air oven. The results can beinferred from Table 1.

EXAMPLE 3

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof triethanolamine dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 1.2 g of NM 4266-750silicone oil (commercial product of HÜLS Silicone GmbH, Nünchritz) isapplied at 1000 rpm. The product thus treated is postcured for 30minutes at 190° C. in a circulating air oven. The results can beinferred from Table 1.

EXAMPLE 4

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 1.2 g of NM 4266-750silicone oil (commercial product of HÜLS Silicone GmbH, Nünchritz) isapplied at 1000 rpm. The product thus treated is postcured for 30minutes at 190° C. in a circulating air oven. The results can beinferred from Table 1.

EXAMPLE 5

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 2.25 g of NM 4266-750silicone oil (commercial product of HÜLS Silicone GmbH, Nünchritz) isapplied at 1000 rpm. The product thus treated is postcured for 30minutes at 190° C. in a circulating air oven. The results can beinferred from Table 1.

EXAMPLE 6

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and coated at 1000 rpmwith an emulsion consisting of 0.75 g of ethylene carbonate, 2.25 g ofwater and 0.3 g of Magnasoft HSSD silicone oil (commercial product ofOSi, Düsseldorf). The product thus treated is postcured for 30 minutesat 190° C. in a circulating air oven. The results can be inferred fromTable 1 .

EXAMPLE 7

150 g of a powdered superabsorber according to Comparative example V1 ischarged in a vertically arranged laboratory mixer with high-speed knivesand treated with 0.3 g of NM 4266-750 silicone oil (product of HÜlsSilicone GmbH, Nünchritz) at 1000 rpm. The product thus produced ispost-cured for 30 minutes at 190° C. in a circulating air oven. Theresults can be inferred from Table 1.

EXAMPLE 8

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and added with 0.75 gof ethylene carbonate dissolved in a clear, transparent emulsionconsisting of 2.25 g of water, 0.15 g of acetic acid (60%), 0.3 g of NM4266-1000 silicone oil (product of Hüls Silicone GmbH, Nünchritz) and 6g of acetone at 1000 rpm. The product thus treated is postcured for 30minutes at 190° C. in a circulating air oven. The results can beinferred from Table 1.

EXAMPLE 9

2.0000±0.0005 g of cellulose is weighed on an analytical balance. Thecellulose is used to form three cellulose plies. One cellulose ply isplaced onto the analytical balance. Subsequently, 0.6000±0.0005 g ofsuperabsorber according to Example 1 is sprinkled over the cellulose plyas uniformly as possible. The second cellulose ply is placed on top, sothat a cellulose/SAP/cellulose sandwich is formed. Again, 0.600 g±0.0005g of superabsorber according to Example 1 is sprinkled thereon asuniformly as possible. The third cellulose ply is placed on top. Theamount of superabsorber used and the number of SAP plies are recorded.

The pad thus prepared is subsequently examined in the FCAT testingapparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

EXAMPLE 10

A superabsorber-cellulose pad is prepared according to the embodiment ofExample 9 but using the superabsorber of Example 2.

Subsequently, the pad is examined in the FCAT testing apparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

EXAMPLE 11

A superabsorber-cellulose pad is prepared according to the embodiment ofExample 9 but using the superabsorber of Example 5.

Subsequently, the pad is examined in the FCAT testing apparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

EXAMPLE 12

A superabsorber-cellulose pad is prepared according to the embodiment ofExample 9 but using the superabsorber of Example 6.

Subsequently, the pad is examined in the FCAT testing apparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

COMPARATIVE EXAMPLE 1

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. The product thus treated is then postcuredfor 30 minutes at l90° C. in a circulating air oven. The results can beinferred from Table 1.

COMPARATIVE EXAMPLE 2

150 g of a powdered superabsorber polymer A is charged in a verticallyarranged laboratory mixer with high-speed knives and initially, 0.75 gof ethylene carbonate dissolved in a mixture of 3 g of water and 6 g ofacetone is added at 1000 rpm. Subsequently, 1.8 g of AP 500 silicone oil(Wacker GmbH) is applied at 1000 rpm. The product thus treated ispostcured for 30 minutes at 190° C. in a circulating air oven. Theresults can be inferred from Table 1.

COMPARATIVE EXAMPLE 3

150 g of the powdered superabsorber from Comparative example 1 istreated with 1.2 g of NM 4266-750 silicone oil in a vertically arrangedlaboratory mixer with high-speed knives at 1000 rpm. The swellingproperties of the product thus treated are determined without furtherheating. The results can be inferred from Table 1.

COMPARATIVE EXAMPLE 4

150 g of the powdered superabsorber polymer A is coated with 1.2 g ofsilicone oil NM 4266-750 in a vertically arranged laboratory mixer withhigh-speed knives at 1000 rpm. The product thus treated is thenpostcured for 30 minutes at 190° C. in a circulating air oven. Theresults can be inferred from Table 1.

COMPARATIVE EXAMPLE 5

A superabsorber-cellulose pad is prepared according to the embodiment ofExample 9 but using the superabsorber of Comparative example 1.

Subsequently, the pad is examined in the FCAT testing apparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

COMPARATIVE EXAMPLE 6

A superabsorber-cellulose pad is prepared according to the embodiment ofExample 9 but using the superabsorber of Comparative example 2.

Subsequently, the pad is examined in the FCAT testing apparatus.

For the blank sample, a pad having 2.0000±0.0005 g of fluff is preparedwithout superabsorber. The result in the form of the recorded curve canbe inferred from FIG. 5.

FIG. 1 illustrates the dependence of the swelling behavior on the amountof functional silicone oil to be used according to the invention,wherein the absorption of the products under free swelling conditions(large excess of solvent, swelling without pressure) is plotted versusthe swelling period.

The products 1, 2, 4, and 5 were treated with varying amounts offunctional silicone oils to be used according to the invention. Theswelling rate of the products is markedly dependent on the percentage ofhydrophobic polymer. Therefore, the absorption rate of the highlyswellable polymers can be controlled directly through the amount ofhydrophobic polymer.

Product V4 shows an absorption behavior roughly corresponding to that ofthe product according to Example 1 (FIG. 2). Product 1 was preparedusing only 0.1 wt.-%, i.e., one eighth of the amount of functionalsilicone oil to be used according to the invention employed in V4,thereby achieving a markedly improved flowability compared to product V4(product 1: 11.5 g/s, V4: 7.7 g/s, cf., Table 1). Thus, the flowabilityof product 1 comes close to the optimum flowability of the uncoatedreference product V1 (12 g/s).

Comparative example V4 shows that even without using a reactivecomponent resulting in additional crosslinking of the polymer particlesat their surface (so-called post-crosslinkers), there is a specificeffect on the absorption rate. However, this effect is comparativelysmall. As a result of the considerably larger amounts, e.g., of siliconeoil that would be necessary, products produced in this manner, having anabsorption rate within the preferred range, are lacking flowability,which is undesirable.

FIG. 3 shows that when using a hydrophobic polymer without functionalgroups (Comparative example V2: polydimethylsiloxane) under conditionsrequired for the production of highly swellable polymers havingcontrolled absorption rate, no substantial deviation from the swellingbehavior of a highly swellable polymer not coated with hydrophobicpolymers is achieved (Example V1). Similarly, such behavior is rated asan indication that the hydrophobic polymer must be fixed at the surfaceof the highly swellable polymer particles via functional groups.

FIG. 4 illustrates the difference in swelling behavior of two productshaving the same amount of functional silicone oil to be used accordingto the invention, where the product from Example 4 has been postcured at190° C. while the product from Comparative example V3 has not beenheated.

Comparative example V1 relates to the surface-post-crosslinked highlyswellable polymer produced without the functional silicone oil to beused according to the invention.

The two comparative products V1 and V3 show a conforming absorptionbehavior, while the product from Example 4 exhibits a significantly moregradual absorption curve. Without intending to limit the scope of thepresent invention, the assumption is made that in order to generateproducts having the desired absorption behavior and acceptableflowability (≧8 gls), a fixation of the functional silicone oils to beused according to the invention at the surface of the hydrophilic,highly swellable polymer is necessary, which is achieved by thepostcuring step.

The property of controlled retardation of the absorption rate isretained even in that case where the superabsorber particles are presentseparated from each other and fixed in a cotton pad. Thus, the change inswelling behavior by hydrophobic coating with polymers according to theinvention is a property of the individual particle and not a result ofagglomeration of individual particles into a hydrophobic agglomeratehaving a reduced surface area.

The fluff combination absorption test is useful as an experiment forexamining the swelling rate of particles separated from each other. Inthis test, a pad consisting of highly swellable polymers and cottonfluff is prepared and exposed to pressure. The pressure may be variedbetween 7 g/cm² and 70 g/cm². Higher pressures are also possible.Subsequently, supply of test solution is allowed to proceed fromunderneath without pressure. In this test, the capillary forces of thecotton fluff have a wetting effect on the individual particles. Theabsorption of liquid over time is recorded and converted into anabsorption curve (cf., FIG. 5). A comparison of the absorption curves ofV5 (including superabsorber V1 with no hydrophobizing), V6 (includingsuperabsorber V2, hydrophobized with a non-reactive, purepolydimethylsiloxane), and the products of the invention according toExamples 9-12 (containing the superabsorbers 1, 2, 5, and 6) clearlydemonstrates the retardation of the absorption rate of highly swellablepolymers having an aminosilicone coating.

This property of controlled retardation of the absorption rate is incontrast to the product property relating to an improvement of theabsorption under pressure with good “initial absorbency” as described inEP-A 705,643.

The Examples 2 through 5 in EP-A 705,643 show that the absorption ofsuperabsorbent polymers that were treated with silicone oil includingamino or epoxy functional groups is higher after 10 minutes both priorto and after postcuring of the materials and thus, the absorption rateis higher than that of materials according to the Comparative examples 1through 8, which were not surface-treated. However, the presentinvention is directed to the opposite of improving the absorptionshortly after the onset of swelling. Rather, the advantage of the highlyswellable polymers of the invention is that a significant improvement ofthe liquid distribution within the absorptive core of a hygienic articleis achieved as a result of the retardation of superabsorber swelling.

TABLE 1 Additive Example Amount Time [minutes] Flowability No.Designation [wt. − % SAP] 0 5 10 15 30 60 120 g/s 1 NM 4266-750 0.1 0 918.5 25.5 34.5 39.5 41 11.5 0.1 0% 22% 45% 62% 84% 96% 100% 2 NM4266-750 0.2 0 6 14 20.5 33.5 42.5 45 10.5 0.2 0% 13% 31% 46% 74% 94%100% 3 NM 4266-750 0.8 0 5 10 16 27 35 37.5 8 0.8 0% 13% 27% 43% 72% 93%100% 4 NM 4266-750 0.8 0.0 3.0 6.0 8.5 16.0 31.0 39.0 8 0.8 0% 8% 15%22% 41% 79% 100% 5 NM 4266-750 1.5 0 2 4 6 12.5 28.5 38 5.5 1.5 0% 5%11% 16% 33% 75% 100% 6 Magnasoft HSSD 0.2 0 11 21 27 34.5 38 39 12 0.20% 28% 54% 69% 88% 97% 100% 7 NM 4266-750 0.2 0 8 16.5 20.5 29 35 3712.1 0.2 0% 22% 45% 55% 78% 95% 100% 8 NM 4266-1000 0.2 0 6.5 14.5 20.530 34.5 36.5 12.4 0.2 0% 18% 40% 56% 82% 95% 100% V1 None 0 0 19.5 29.534 39.5 42 42.5 12 0 0% 46% 69% 80% 93% 99% 100% V2 AP 500 silicone oil1.2 0 16.5 26 30.5 36 39 39.5 6.5 1.2 0% 42% 66% 77% 91% 99% 100% V3 NM4266-750 0.8 0 18.5 26.5 31.5 37.5 40.8 42 8 0.8 0% 44% 63% 75% 89% 97%100% V4 NM 4266-750 0.8 0.0 12 20 28 39 44 45 7.7 0.8 0% 27% 44% 62% 87%98% 100% Absorption data in g/g or %

TABLE 2 Example Time [minutes] Time factor 1 20 2 2 27 2.7 3 29 2.9 4 525.2 5 56 5.6 6 15.5 1.55 7 24.5 2.45 8 22.5 2.25 V1 10 1 V2 12 1.2 V3 131.3 V4 19.5 1.95

What is claimed is:
 1. A swellable polymer capable of absorbing aqueousliquids, comprised of a) monomers bearing monoethylenically unsaturatedacid groups, b) optionally, other monomers copolymerizable therewith,and c) optionally, water-soluble polymers suitable as basis forgrafting, and d) at least di-unsaturated monomers acting ascrosslinkers, wherein the polymer has been coated with e) a reactive,water-insoluble hydrophobic polymer of general formula 1

wherein n=50-99 m=1-50 k=1-11 i=1-12, and R═H, alkyl, hydroxyalkyl,aminoalkyl, R′═H, alkyl, hydroxyalkyl, aminoalkyl, f) another reactivecomponent capable of reacting with carboxyl groups or carboxylate anionsto form additional crosslinking sites on the particle surface, andpostcured.
 2. The polymer according to claim 1, wherein the hydrophobicpolymer according to formula 1 is additionally provided with polyetherside-chains on the polysiloxane backbone.
 3. The polymer according toclaim 1, wherein they contain the hydrophobic polymers e) in amounts offrom 0.005 to 2 wt.-%, relative to the polymer from a) through d). 4.The polymer according to claim 1, which contains ethylene carbonateand/or polyhydric alcohols as component f).
 5. The polymer according toclaim 4, which contains ethylene carbonate and/or polyhydric alcohols ascomponent f).
 6. The polymer of claim 1, wherein said postcuring iseffected within a temperature range of from 80 to 230° C.
 7. The polymerof claim 1, which have a flowability of more than or equal to 8 g/s,measured according to DIN
 53492. 8. The polymers of claim 1, wherein 70%of the maximum absorptive capacity of the superabsorbent polymer willnot be reached after only 10 minutes but, at the earliest, after a 15minutes swelling time without pressure load.
 9. A process for theproduction of the highly swellable polymer according to claim 1, whichabsorb aqueous liquids, characterized in that a swellable polymerabsorbing aqueous liquids, constituted of a) monomers bearingmonoethylenically unsaturated acid groups, b) optionally, other monomerscopolymerizable therewith, and c) optionally, water-soluble polymerssuitable as basis for grafting, and d) at least di-unsaturated monomersacting as crosslinkers, characterized in that the polymers have beencoated with e) a reactive, water-insoluble hydrophobic polymer ofgeneral formula 1

wherein n=50-99 m=1-50 k=1-11 i=1-12, and R═H, alkyl, hydroxyalkyl,aminoalkyl, R′═H, alkyl, hydroxyalkyl, aminoalkyl, and f) at least oneother reactive component capable of reacting with carboxyl groups orcarboxylate anions to form additional crosslinking sites on the particlesurface, and subsequently subjected to a thermal treatment in atemperature range of from 80 to 230° C.
 10. The process according toclaim 9, wherein the hydrophobic polymer according to formula 1 has beenprovided with additional polyether side-chains on the polysiloxanebackbone prior to the reaction.
 11. The process according to claim 9,wherein the hydrophobic polymers e) are used in amounts of from 0.005 to2 wt.-%, relative to the swellable polymer constituted of a) through d).12. The process according to claim 9, wherein the reactive,water-insoluble hydrophobic polymer e) is used in its protonated form.13. The process according to claim 9, wherein the hydrophobic polymer e)in its protonated form is applied onto the swellable polymer from a)through d) together with the multi-functional compound f) as a solutionor emulsion in water.
 14. The process according to claim 9, wherein theswellable polymer constituted of a) through d) is coated with themulti-functional reactive compound f) prior to, simultaneously with, orafter coating with the hydrophobic, reactive polymer e).
 15. The processaccording to claim 9, wherein each coating step in the separate coatingof the swellable polymer from a) through d) with reactive hydrophobicpolymer e) and multifunctional compound f) is followed by a postcuringstep.
 16. The process according to claim 9, wherein merely a subsequentheat treatment is performed in the simultaneous coating of the swellablepolymer from a) through d) with hydrophobic polymer e) andmultifunctional compound f).
 17. A method of using the polymersaccording to claim 9, by incorporting said polymers into hygienearticles for absorbing body fluids.